Most Helpful Guy
The first time you go through his things, you've already lost. You are then the villain of the piece, regardless of anything you find.
There is a simple way to know - ask him politely to stop flirting with some girl. (Not all girls, just pick one to do your test on.)
If he does, then he is still more concerned about your feelings than another girls... if he doesn't, then you may have an issue. Because you're not trying to control him or tell him he can't have any female friends... you're just asking him politely to stop flirting with one particular girl.
Honestly though? I think you've already destroyed this, with your obvious mistrust. You're just too possessive and insecure for an adult relationship right now.